I live less than 2 hours from St. Louis and we go all the time. When we stay overnight we almost always stay at the Embassy Suites in St. Charles (its a little bit of a drive but right next to maryland heights where you mentioned) - the hotel is clean, price is reasonable and the rooms are ample, most being suites priced around 120... but that includes a great breakfast that rivals some of the restaurants in the area. We have also stayed at the Days Inn on Linbergh which is a great value. They are close to Grants Farm and the Magic House, have a playground inside the hotel along with a good size pool. They also have rooms that sleep 6 (3 queens) and almost always come in under 100.
There is sooo much to do in St. Louis and much of it is free or really inexpensive. Grants Farm, Magic House (if you decide to do this one lmk and I can send you a coupon - pretty sure its bogo), City Museum, Science Center, Zoo, Butterfly House, Lacledes Landing and Six Flags (although that's totally overrated). There's also Dave & Busters which is a lot of fun but you will be paying out the nose there. Mills Mall is pretty cool (outlet mall with tons in it for kids) and depending on your sense of humor The Royal Dumpe at Laclede's Landing is a great dinner show. We go to the Royal Dumpe for dinner, walk Lacledes Landing a bit then take a horse drawn carriage ride which is great at night. Tucanos in St. Charles is pretty good food wise but not overly cheap (Similar to Texas de Brazil or Fogo de Chao).
If you are into outdoorsy stuff and will be driving or renting a car and want to drive some there is the Cahokia Mounds in Illinois (20-45mins depending on where you stay), and if you want to go even further into Southern Illinois (1-2hrs) there's Garden of the Gods, Cave in the Rock, Fern Clyffe, Elizabethtown ferry and much much more.
